This application is based on and claims priority from Japanese Patent Application Nos. HEI 3-252687 filed Jun. 26, 1991 and HEI 4-101383 filed Apr. 21, 1992, the disclosure of which is incorporated by reference herein.
The present invention relates to a scanning optical system such as a laser printer or a laser photoplotter and, more particularly, to a system having a monitoring unit that is capable of indirect detection of the position in which a laser beam spot is scanned over the image plane.
Some conventional models of scanning optical system that require high patterning or imaging precision are equipped with a monitoring optical unit for detecting the position in which a laser beam spot is scanned over the image plane. The conventional monitoring optical units are available in various types; in one type, part of the writing beam is isolated by a half mirror and used for monitoring purposes; in another type, the output light is divided into two beamlets by means of a polarizing beam splitter; and in still another type, the writing beam and the monitoring beam are allowed to fall on a polygonal mirror at different angles of incidence with respect to the sub-scanning direction, whereby the two light beams are spatially separated.
However, the respective types have their own problems. The optical unit using a half mirror is unable to produce a monitor signal, when patterning or imaging is not done. With the unit that uses a polarizing beam splitter, perfect isolation of polarized beams is impossible. In addition, since the polarizing performance is varied in accordance with the angle of incidence of the beam relative to the polarizing splitter, the quality to the written image would be adversely affected. The unit for achieving spatial isolation between the writing beam and the monitoring beam has the problem of curvature in the scanning lines of the monitoring beam. In addition, efficient isolation is difficult if a scanning lens of small F number is to be used.